Process for the preparation of isatoic anhydride

ABSTRACT

Reacting, in a first step, an aqueous alkali metal salt solution of phthalic acid monoamide, optionally in admixture with phthalimide, under alkaline conditions with alkali metal hypochlorite, e.g., at a temperature of about -5* to +40* C., and adding all at once, in a second step, an amount of acid substantially equivalent to the alkali present in the solution to neutralize such alkali and form isatoic anhydride and thereby cause separation of the isatoic anhydride from the solution, said isatoic anhydride being a known intermediate for the preparation of plant protection agents and dyestuffs.

United States Patent Humburger et al.

[ 51 Aug. 29, 1972 [S4] PROCESS FOR THE PREPARATION OF ISATOIC ANHYDRIDE[73] Assignee: Farbenfabriken Bayer Aktiengesellschaft, Leverkusen,Germany 22 Filed: May 19,1969

21 Appl.No.: 825,890

[30] Foreign Application Priority Data May 20, 1968 Germany ..P 17 70458.4

[52] US. Cl. ..260/244 A, 260/518 [51] Int. Cl. ..C07d 87/10 [58] Fieldof Search ..260/244 A [56] References Cited UNITED STATES PATENTS3,324,119 6/1967 Hill et al. ..260/244 A- FOREIGN PATENTS ORAPPLICATIONS 127,138 12/1901 Germany ..260/244 A OTHER PUBLICATIONSConant et al., The Chemistry of Organic Compounds, 3rd ed., p. 502,N.Y., MacMillan, 1947. QD25 1.C67 McKee In: Wiley Fiveand Six- MemberedCompounds with Nitrogen and Oxygen, p. 364, N.Y., Interscience- Wiley,1962. QD401.FS8

Primary ExaminerNatalie Trousof Attorney-Burgess, Dinklage & SprungABSTRACT Reacting, in a first step, an aqueous alkali metal saltsolution of phthalic acid monoamide, optionally in ad- 10 Claims, 2Drawing Figures PATENTED M1829 m2 ISATOIC ANHYDRIDE FROM PHTHAL/C ACIDMONOAMIDE (WITH EQUIVALENT AMOUNTS OF NaOH AND PHTHAL/M/DE AND MONOAMIDE CONTENT YIELD YIELD ISATOIC ANHYDRIDE ISATOIC ANHYDRIDE PHTHALIMIDE'0 x x 3L x x x J i x y/ 60- y FIG. I m

0/ CONTENT PHTHAL/MIDE (ACCORDING TO TITRAT/ON) 0 2O 2'0 3'0 42) a0 20090 a0 70 50 50 40 30 2O .10 0

PHTHAL/M/DE PH THAL/C ACID MONOAM/DE ISATOIC ANHYDR/DE FROM PHTHAL/M/DE(ALKAL, SOLUTION STORED AT 3C) (1.05 MOL NaOH PER MOL PHTHALIMIDE)CONTENT YIELD 100- lSATO/C ANHYDR/DE YIELD ISATOIC ANHYDR/DE PHTHAL/MIDE2 OO- v Y ,OONTENT PHTHAL/M/DE (MEASUREMENT) "u 40- cONTENT PHTHALIM/DEFIG. 2 (TlTRZT/ON) 0. 0 0 2 4 5 a 70 72 14 I6 18 20 22 u gflg INVENTORS:

SIEGBERT HUMBURGER, WILLI mm.

. HTTDAA/f/S PROCESS FOR THE PREPARATHON OF ISATOIC ANHYDRKDE Thepresent invention relates to and has for its objects the provision forparticular new methods of reacting phthalic acid monoamide, optionallyin admixture with phthalimide, i.e., in the form of the correspondingaqueous alkali metal salt solution under alkaline conditions, withalkali metal hypochlorite, and thereafter neutralizing with acid,whereby to produce isatoic anhydride, which is a known intermediate fororganic syntheses, e. g., in a simple manner, using readily availablestarting materials and resulting in outstanding yields and purity, withother and further objects of the invention becoming apparent from astudy of the within specification and accompanying examples.

It is known that phthalic acid monoamide (i.e.,ortho-aminocarbonyl-benzoic acid) can be converted into anthranilic acid(ortho-amino-benzoic acid) by reaction with alkali metal hypochlorite inalkaline aqueous solution at elevated temperatures with an excess ofalkali'and without neutralizing the mixture according to the mechanismof the Hoffmann acid amide degradation. Anthranilic acid can, in turn,be reacted with phosgene to give isatoic anhydride, which is thereforeaccessible in this manner in a 2-step process.

On the other hand, German Pat. (Deutsches Reichspatent) No. 127,138describes a method for the preparation of isatoic anhydride fromphthalic acid imide (pthalimide) and hypochlorite without the use ofphosgene. However, it is difficult, according to the particulars givenin such German patent, to prepare isatoic anhydride reproducibly and ingood yields, since the reaction is very complex and leads to theformation of by-products [see Mohr, Journal fur Praktische Chemie (2)80, l-3l (1909)]. Because of these difficulties, the process of GermanPat. (Deutsches Reichspatent) No. 127,138 has not become established inindustry.

More recently, a specific embodiment of said German Patent method ofpreparation has been proposed which comprises dissolving phthalic acidimide in aqueous solution of alkali to form the corresponding alkaliphthalimide salt and then reacting such phthalimide salt immediatelythereafter with alkali metal hypochlorite solution; a narrowlyrestricted pH range (i.e., acid pH) must be maintained by addition ofacid (see U.S.Pat. No. 3,324,119).

This last-mentioned process, however, only affords good yields if thephthalimide salt is still present in substantially unchanged from in thestarting mixture upon the reaction with the alkali metal hypochloritesolution. According to such US. Pat. No. 3,324,119, the presence of atleast 50 percent, and for the achievement of good yields at least 80percent, of unchanged phthalimide salt is necessary in the reactionmixture. It will be realized that the reaction of the phthalimide saltsolution must be effected immediately after it has been prepared becausesuch salt is readily hydrolyzed to phthalic acid monoamide at theinherently alkaline pH value of the aqueous salt solution in question.

It has now been found, in accordance with the present invention, that aversatile and smooth process may be provided for the production infavorably higher yields and higher purity of isatoic anhydride, in atechnologically simple manner and without the use of phosgene, whichcomprises reacting, in a first step, an

aqueous alkali metal salt solution of phthalic acid monoamide, i.e.,aqueous alkali metal phthalic acid monoamide salt solution, optionallyin admixture with phthalimide, under alkaline conditions, i.e., atalkaline or basic pH-range above 10, preferably above 12 with alkalimetal hypochlorite and adding all at once, in a second step, an amountof acid approximately, e.g., substantially, equivalent to the alkalipresent in the solution, to neutralize such alkali, whereby the isatoicanhydride separates out of the solution and may be recovered.

It is most surprising that, by means of the instant process, isatoicanhydride can be obtained in good yields. From the teachings of said US.Pat. No.

3,324,119, it would have been expected that solutions of alkali metalsalts of phthalic acid monoamide would not be suitable for thepreparation of isatoic anhydride by reaction with alkali metalhypochlorite under alkaline conditions. From the other prior artmentioned above, it would have been expected that such solutions wouldhave to be degraded with alkali metal hypochlorite to give anthranilicacid.

One of the chief advantages of the production process of the presentinvention as compared with the prior art is that it is not necessary totake complicated and expensive precautions to ensure that the startingmaterial, e.g., phthalimide salt, is unchanged. Such precautions areespecially necessary in the known process according to said US. Pat. No.3,324,1 19 where the presence of a substantial proportion of phthalicacid monamide in the phthalimide is said to prevent a reasonable yield,and where such presence is very likely, in the absence of suchprecautions, because phthalic acid imide is readily hydrolyzed inalkalineaqueous solution.

In carrying out the production process of the present invention, one canstart advantageously from preformed phthalic acid monoamide solutions,or one can use phthalic acid imide solutions without having to take intoaccount their degree of hydrolysis. This has considerable technologicaland economic advantages, since it is very difficult to so dissolvephthalic acid imide in stoichiometric amounts of solution of alkali, andto so handle such solution, that no, or only slight, hydrolysis occurs.

This becomes understandable when the hydrolysis speeds are considered:according to US. Pat. No. 3,324,1 19, a sodium phthalimide solution ishydrolyzed to the extent of 10 percent after 35 minutes at 0 C., andafter only 2 minutes at 30 C. A 50 percent hydrolysis, and thususelessness of the solution for the process of US. Pat. No. 3,324,119,results after only minutes at 0 C., or after only 23 minutes at 30 C.

The starting alkali metal salt solutions of phthalic acid monoamide(optionally in admixture with phthalimide) which are suitable for theproduction process according to the present invention can be prepared byvarious known methods, as the artisan will appreciate. Thus, phthalicanhydride can be reacted with ammonia in aqueous-alkaline medium to givethe monoamide, which is obtained as the corresponding alkali metal saltsolution. Also, phthalic acid imide dissolves in aqueous alkali metalsolutions with the formation of the appropriate alkali metal saltswhich, as mentioned above, are subject at the alkaline pH of thesolution to a rapid hydrolysis to give the monoamide. For economicreasons, the sodium salt is preferably used although of coursecorresponding salt solutions of other alkali metals (e.g., potassium,lithium, etc.) are also usable.

Alkali metal hypochlorite solutions which are usable in the instantprocess advantageously may be cheap commercial grade products which, inthe simplest case, are obtained from chlorine and alkali metalhydroxide. Here again, the inexpensive sodium compound is of chiefinterest, although potassium, lithium, etc, hypochlorite solutions arealso usable.

A minimum proportion of the alkali, i.e., alkali metal, present in theinitial mixture compared with the phthalic acid monoamide or imide isessential for the success of the alkali metal hypochlorite reaction.There must be present at least 0.9 mol, preferably about l.0l.5 mols, ofalkali metal hydroxide (e.g., NaOl-I, KOI-l, LiOI-l, etc.) per mol ofthe monoamide or imide. However, larger amounts of alkali metalhydroxide may also be favorable, particularly when pure monoamide isused, for example up to about 4 mols per mol of monoamide or imide. Ingeneral, substantially between about 0.9-4 mols of alkali, i.e., alkalimetal hydroxide, are used per mol of the monoamide or imide.

Advantageously, the alkali metal hydroxide to be used may, in part, bepresent in the amide solution or imide solution and the other part maybe added with the alkali metal hypochlorite solution.

The amount of acid used in the second step according to the process ofthe present invention must be approximately equivalent to the amount ofalkali present for desired neutralization, as the artisan willappreciate. To achieve good yields, only slight deviations from theequivalent amount are admissible, for example up to about 10 percentless or an excess of about 5 percent, i.e., mol percent. In general,substantially between about 0.9 to 1.05 mol equivalents of acid per molof alkali, i.e., alkali metal hydroxide present, should be used. Theaddition of still more acid should be avoided. The nature of the acidhas, as such, no decisive influence on the course of the reaction, justits role as neutralizing agent. For economic reasons, strong mineral orinorganic acids, for example sulfuric acid, hydrochloric acid, nitricacid, phosphoric acid, or the like, are generally used.

The composition of the solution of phthalic acid monoamide or itsmixture with phthalimide can be varied within wide limits. It issurprising and an important feature of the present invention that evenmonoamide solutions with an exceptionally low proportion of phthalimidecan be reacted to give the same yields of isatoic anhydride whichaccording to the prior art are said to be attainable only with pure orvery slightly hydrolyzed imide solutions. In this regard, it has provedadvantageous to use alkali metal salt solutions of mixtures of phthalicacid monoamide and phthalimide with a content of phthalimide ofsubstantially between about 5-50 percent, in particular between about-30 mol percent of the mixture, the remainder being correspondingly95-50 percent, in particular 9070 mol percent of the monoamide.

FIG. 1 shows graphically the dependence of the yields of isatoicanhydride on the composition of the starting solutions.

FIG. 2 shows graphically the fact that alkaline phthalimide solutions inthe sense of the present invention can be stored advantageously overlonger periods before the reaction with alkali metal hypochlorite,without lessening of the yield. The graph of FIG. 2 shows the variationof the yield with time (in hours), as well as the variation of thedegree of hydrolysis of the phthalimide solution with time. The isatoicanhydride is prepared by following the procedure according to Example 2.The hydrolysis curve was determined according to the particulars givenin said U.S. Pat. No. 3,324,1 l9, and by an ultra-violet radiationmethod.

The duration of the first step of the production process of the presentinvention is important. It is closely coupled with the reactiontemperature. To avoid side reactions, the reaction is expedientlycarried out at low temperature. A lower limit is given by the freezingtemperature of the solution or the temperature at which alkali metalsalts crystallizing out would make the process industrially impractical.Since the reaction is strongly exothermic, the maintenance of a lowtemperature makes cooling necessary. The upper temperature limit dependsin practice on the degree of cooling which can be effected. Taking thesefacts into account, a temperature range of substantially between about 5to about +40 C. may be regarded as suitable. High temperatures implyshort reaction times, lower temperatures require longer reaction times.For the temperature range from about 0 to 20 C., for instance, residenceperiods of correspondingly about l0 minutes to about 10 seconds haveproved suitable, as specifically disclosed in Example 5.

The second step of the production process according to the presentinvention serves for the completion of the reaction and for theseparation of the isatoic anhydride in solid form. The temperatureduring and after the addition of the equivalent amount of acid may bevaried between wide limits. The range from substantially between about10 C. to about C., particularly from about 25 to 50 C., has provedsuitable. Here, too, the reaction time depends on the temperature. Inthe lower part of the range mentioned, about half an hour may benecessary for the complete separation of the isatoic anhydride. Athigher temperatures, this time shortens to a few minutes, e.g., about 5minutes. Since the separated isatoic anhydride is relatively stableunder the reaction conditions, it may, especially when the lower part ofthe temperature range is used, remain in the reaction mixture for alonger period before isolation, for example for about 3-5 hours.

The production process of the present invention can, in principle, becarried out batch-wise or continuously. The exact observance of thereaction conditions, especially the temperatures and residence periods,which is a precondition for the attainment of good yields, can best beaccomplished with continuous working methods. Continuous operation istherefore preferred for the industrial working of the process. Suchcontinuous operation may be effected by a method in which the aqueousalkali metal salt solution of phthalic acid monoamide, optionally inadmixture with phthalimide salt, is conveyed from a storage container bymeans of a metering pump into a mixing chamber at the desired rate and,simultaneously, the appropriate amount of an alkali metal hypochloritesolution which may contain alkali metal hydroxide is continuouslymetered in. After the mixing chamber there follows a residence zone inwhich the mixture can react in the sense of the first step of theprocess according to the present invention. By adapting the throughputamounts to the dimensions of the reaction chamber, the average residenceperiod necessary in the alkaline range can be exactly adjusted. Beyondthis reaction zone, addition of acid takes place in the sense of thesecond step of the instant process, again continuously and adapted tothe amount of alkali present. The acid mixing apparatus may comprise forexample, a stirrer vessel or a circulating apparatus which is sodimensioned that the residence time required for the complete separationof the isatoic anhydride formed is achieved. The isatoic anhydrideobtained as suspension can then be filtered batch-wise or continuouslyby known methods. It is obtained in a readily filterable form which iseasy to wash free from salt and can, in general, be immediately used forany further processing.

An important advantage of this continuous form of operation is that onecan use phthalic acid monoamide salt solutions or a mixture withphthalimide salt with relatively low phthalimide content. Thesesolutions can without further operations be used as stock solutions;whereas the high-grade phthalimide salt solutions necessary for theprocess of said U.S. Pat. No. 3,324,119 cannot, or can only withconsiderable expense, be held in stock, since even during the process ofdissolving, and during storage, even at low temperature, considerablehydrolysis occurs as discussed above.

The isatoic anhydride which can be prepared according to the presentinvention in a technologically simple manner with good yields and highpurity is a valuable intermediate for organic syntheses, for example forthe preparation of plant protection agents and dyestuffs, as the artisanwill appreciate.

Thus, the present invention contemplates, in particular, a process forthe production of isatoic anhydride which comprises reacting, in a firststep, an aqueous alkali metal salt solution of a member selected fromthe group consisting of phthalic acid monoamide, and a mixture ofphthalic acid semi-amide and phthalimide, in the presence ofsubstantially between about 0.9 4 mols of alkali metal hydroxide per molof said member, with alkali metal hypochlorite at a temperaturesubstantially between about -5 to +40 C., and adding all at once, in asecond step, substantially between about 0.9 to 1.05 mol equivalents ofmineral acid per mol of alkali metal hydroxide present in the solutionat a temperature substantially between about to 80 C. to neutralize suchalkali metal hydroxide, whereby such isatoic anhydride forms andprecipitates from the solution.

Preferably, the first step is carried out at a temperature substantiallybetween about 0 to 20 C. for a residence period of correspondinglybetween about 10 minutes to 10 seconds.

In particular, the aqueous alkali metal salt solution is a solution of amember selected from the group consisting of phthalic acid monoamide,and a mixture of 95-50 mol percent phthalic acid monoamide and 5-50 molpercent phthalimide. Furthermore, especially advantageous from aneconomical standpoint is the case where the aqueous alkali metal saltsolution is the corresponding sodium salt solution, the hypochlorite issodium hypochlorite, the alkali metal hydroxide is sodium hydroxide, andthe acid is a mineral acid such as hydrochloric and/or sulfuric acid.

The production process of the present invention is illustrated, withoutlimitation, by the following examples.

EXAMPLE la [Sodium phthalic acid monoamide] 148.1 g (1 mol) phthalicanhydride are added to a warm (30 C.) mixture of 174.5 ml of water and159 ml (2.12 mols) of 25 percent aqueous ammonia solution. When all theanhydride is dissolved (pH 7), cooling to 50 C. is effected and afurther l48.lg (1 mol) of phthalic anhydride are added. Thereafter,110.5 ml of 48 percent aqueous NaOH (2 mols) are added. The temperaturerises to over C. and a clear solution of pH 9 is obtained. Cooling to 10C. is then effected.

EXAMPLE lb [Isatoic anhydride] A mixture of 144.5g (0.2 mol) ofhypochlorite solution with 10.3 percent NaOCl and 0.55 percent NaOH andml (0.6 mol) aqueous solution of sodium hydroxide with 200 g/l NaOH isadded dropwise in 3 minutes at 10 C., with stirring, to 140.7g of anaqueous solution with 37.4g (0.2 mol) sodium phthalic acid monoamide and0.4g (0.01 mol) NaOH prepared according to Example 1a above. ml (0.713mol) of aqueous hydrochloric acid with 200 g/l HCl are then added in onepouring; heating to 35 C. is effected, followed by stirring for 30minutes. This is followed by suction filtration, washing, and drying for10 hours at 50-55 C.

Yield: 22.6g with 96.2 percent (purity) isatoic anhydride correspondingto 67 percent of the theory.

EXAMPLE 2 29.4g (0.2 mol) phthalimide are dissolved during 10 minutes at10 C. in 110 ml (0.22 mol) 2N aqueous solution of sodium hydroxide. Themixture is then left to stand for 16 hours at 3 C. In this solution, 5percent phthalimide is determined by titration, the remainder beingphthalic acid monoamide. 92.5 ml (0.2 mol) hypochlorite solution with161 g/l NaOCl and 7.9 g/l NaOH are then added dropwise in 5 minutes at10 C. and 116 ml (0.119 mol) of aqueous sulfuric acid with 100 g/l H SOare subsequently added in one pouring and the same procedure is followedas in Example lb. Yield: 27.6g with 97.6 percent (purity) isatoicanhydride corresponding to 82.6 percent of the theory.

EXAMPLE 3 2.94g (0.02 mol) phthalimide freshly dissolved at 3 C. in 11ml of 2N aqueous solution of sodium hydroxide are added to 118.35 ml ofan aqueous solution with 33.6g (0.18 mol) sodium phthalic acid monoamide(prepared as in Example la). 11 percent phthalimide is determined bytitration. Further work is then carried out as in Example 2. Yield:28.4g with 96.5 percent (purity) isatoic anhydride corresponding to 84percent of the theory. The same yield is obtained by using a EXAMPLE 414.7g (0.1 mol) phthalimide freshly dissolved at 3 C. in 55 ml of 2Naqueous NaOH are added to 65.75 ml of an aqueous solution with 18.7g(0.1 mol) sodium phthalic acid monoamide (prepared as in Example la). 52percent phthalimide is determined by titration. Further work is thencarried out as in Example 2. Yield: 28.8g with 93.5 percent (purity)isatoic anhydride corresponding to 82.1 percent of the theory.

EXAMPLE Phthalimide is dissolved in 2N aqueous solution of sodiumhydroxide (5-10 percent excess) in a stirrer vessel during a period of20 minutes at 20 25 C., and the aqueous solution is cooled to 3 C. inthe course of about 1 hour. About 40percent of the phthalimide ishydrolyzed in the solution. After a further 3 hours (about 65 percentphthalimide hydrolyzed), the liquid is passed into a mixing apparatusinto which the equivalent amount of a cold (3 C.) hypochlorite solution(with 14 percent NaOCl and 0.6 percent NaOH) is continuously metered bymeans of a metering pump. Pumping in lasts 4 hours; during this time upto about 85 percent of the phthalimide is hydrolyzed. The reac tionmixture passes from the mixing chamber through a cooled residence zonein which, with a residence period determined by the throughput amountsin a limited temperature range, the reaction heat of the first processstep is removed. The water-white solution runs into a stirrer vessel inwhich neutralization is effected by continuous metering in of anequivalent amount of percent aqueous H 80 in the second step of theprocess. By recycling, regulated by means of a level indicator, the warm(about 40 C.) suspension of isatoic anhydride is continuously dischargedonto a suction filter. There is thus achieved an average residenceperiod of about 1 hour in the neutralization vessel. After briefafter-washing, the moist isatoic anhydride can be further processed. Thecontent is determined by diazotization before and after hydrolysis.

In the following Table are listed the pure yields at various residenceperiods and temperatures of the first process step. They were obtainedin a semi-industrial experimental apparatus with a throughput of 13kg/hour.

TABLE Residence period Temperature C Yield isatoic anhydride 5 min. 6C87.6% 4 min. 4C 87.4% 4 min. 6C 87.7% 4 min. 8C 86.1% 4 min. 10C 86.0% 4min. 12C 82.0% 2.9 min. 8C 85.9% 52.5 sec. 4C 93.5% 52.5 sec. 8C 85.6%52.5 sec. 12C 91.5% 52.5 sec. 16C 87.6%

It will be appreciated that the instant specification and examples areset forth by way of illustration and not limitation, and that variousmodifications and changes may be made without departing from the spiritand sco e of he resent invention.

Whatis claimehl 1s:

1. In the production of isatoic anhydride wherein an aqueous solution ofan alkali metal salt of phthalic acid monoamide is reacted with analkali metal hypochlorite to form the corresponding isocyanate and anacid is employed to convert the isocyanate to isatoic anhydride, theimprovement which comprises effecting the reaction with hypochlorite ata pH above about 10 in the presence of an excess of alkali metalhydroxide up to 4 mols per mol of said monoamide, and adding the acidall at once in a subsequent step and in an amount substantiallyequivalent to the alkali present in the solution to neutralize suchalkali whereby isatoic anhydride separates out from the solution.

2. Process according to claim 1 wherein said acid addition is carriedout with substantially between about 0.9 to 1.05 mol equivalents of acidper mol of alkali present in the solution. a

3. Process according to claim 1 wherein said acid addition is carriedout at a temperature substantially between about 10 to C.

4. Process according to claim 1 wherein said reaction is carriedout'at atemperature substantially between about 0 to 20 C. for a residenceperiod of correspondingly between about 10 minutes to 10 seconds.

5. Process according to claim 1 wherein said process is carried outcontinuously.

6. Process according to claim 1 wherein said alkali metal phthalic acidmonoamide salt is the corresponding sodium salt, said alkali metalhypochlorite is sodium hypochlorite, said alkaline conditions aremaintained by the presence of-sodium hydroxide as alkali, and said acidis mineral acid.

7. Process according to claim 6 wherein said acid is selected from thegroup consisting of hydrochloric acid, sulfuric acid and mixturesthereof.

8. Process according to claim 1 wherein said reaction is effected atabove 12 with alkali metal hypochlorite at a temperature substantiallybetween about 5 to +40 C. and said acid addition is effected withbetween about 0.9 to 1.05 mol equivalents of mineral acid per mol ofalkali metal hydroxide present in the solution at a temperaturesubstantially between about 10 to 80 C.

9. Process according to claim 8 wherein said reaction is carried out ata temperature substantially between about 0 to 20 C. for a residenceperiod of correspondingly between about 10 minutes to 10 seconds.

10. Process according to claim 1 wherein said aqueous alkali metal saltsolution is the corresponding sodium salt solution, said alkali metalhypochlorite is sodium hypochlorite, said alkali metal hydroxide issodium hydroxide, and said mineral acid is selected from the groupconsisting of hydrochloric acid and sulfuric acid.

2. Process according to claim 1 wherein said acid addition is carriedout with substantially between about 0.9 to 1.05 mol equivalents of acidper mol of alkali present in the solution.
 3. Process according to claim1 wherein said acid addition is carried out at a temperaturesubstantially between about 10* to 80* C.
 4. Process according to claim1 wherein said reaction is carried out at a temperature substantiallybetween about 0* to 20* C. for a residence period of correspondinglybetween about 10 minutes to 10 seconds.
 5. Process according to claim 1wherein said process is carried out continuously.
 6. Process accordingto claim 1 wherein said alkali metal phthalic acid monoamide salt is thecorresponding sodium salt, said alkali metal hypochlorite is sodiumhypochlorite, said alkaline conditions are maintained by the presence ofsodium hydroxide as alkali, and said acid is mineral acid.
 7. Processaccording to claim 6 wherein said acid is selected from the groupconsisting of hydrochloric acid, sulfuric acid and mixtures thereof. 8.Process according to claim 1 wherein said reaction is effected at above12 with alkali metal hypochlorite at a temperature substantially betweenaboUt -5* to +40* C. and said acid addition is effected with betweenabout 0.9 to 1.05 mol equivalents of mineral acid per mol of alkalimetal hydroxide present in the solution at a temperature substantiallybetween about 10* to 80* C.
 9. Process according to claim 8 wherein saidreaction is carried out at a temperature substantially between about 0*to 20* C. for a residence period of correspondingly between about 10minutes to 10 seconds.
 10. Process according to claim 1 wherein saidaqueous alkali metal salt solution is the corresponding sodium saltsolution, said alkali metal hypochlorite is sodium hypochlorite, saidalkali metal hydroxide is sodium hydroxide, and said mineral acid isselected from the group consisting of hydrochloric acid and sulfuricacid.